Increased myofilament Ca sensitivity is a common feature of sarcomeric mutations that cause familial hypertrophic cardiomyopathy (HCM) and the same Ca sensitivity increase has also been described after myocardial infarction (MI). Both diseases are associated with a high risk for ventricular arrhythmia and sudden death, but the mechanisms linking myofilament Ca sensitivity and arrhythmia susceptibility remain poorly understood. This renewal application will investigate the mechanistic link between myofilament Ca sensitivity and triggered arrhythmia. During the last funding cycle, we discovered that two Ca-sensitizing Troponin T (TnT) mutations (I79N, F110I) and the Ca sensitizing compound EMD57033 both significantly increase the Ca binding affinity of the cytosol (i.e., apparent Kd), but have no effect on the maximal cytosolic buffering capacity (i.e., Bmax). These data establish for the first time a direct effect of the TnT mutations on cytosolic Ca buffering properties. The main consequence of the increased myofilament Ca binding affinity at physiological heart rates was an increase in end-diastolic Ca in the cytosol. Surprisingly, and in contrast to previous predictions, Ca content in the sarcoplasmic reticulum (SR) was NOT reduced. Rather, the net effect of increased myofilament Ca binding was the accumulation of Ca in the cytosol during periods of rapid heart rates, which after brief pauses then led to SR Ca overload, action potential (AP) prolongation and early afterdepolarizations. Together with our finding of pause-dependent triggering of serious ventricular arrhythmia both in HCM and post-MI mouse models, these provocative new results have led us to formulate the following hypothesis: Increased myofilament Ca binding affinity is a fundamental defect that causes pause-dependent SR Ca overload and thereby increases triggered arrhythmia susceptibility. Experiments in Aim 1 will test whether our hypothesis can be generalized to other sarcomeric mutations that cause inherited cardiomyopathies (i.e., familial HCM or dilated cardiomyopathy [DCM]). Aim 2 will determine how increased myofilament Ca binding alters Ca handling and AP regulation in different species. Aim 3 will test whether increased myofilament Ca binding causes arrhythmia risk in acquired heart disease (i.e., ischemic cardiomyopathy). Aim 1. To test the hypothesis that sarcomeric mutations that change myofilament Ca sensitivity cause concordant changes in myofilament Ca binding affinity and cytosolic Ca buffering Aim 2. To determine the effect of increased myofilament Ca binding affinity on myocyte Ca homeostasis and action potential regulation in different species Aim 3. To test the hypothesis that increased myofilament Ca binding contributes to triggered arrhythmia after chronic MI